There is a potential bug that could cause this that has been fixed and will be in the next vSphere-patch.
Until then there is workaround to change this registry key in the virtual desktop:
1) Add the following registry HKLM\\SOFTWARE\\VMware, Inc.\\VMware SVGA DevTap\\NoBlankOnAttach
2) Set the value to 1.
Please try that on 1-2 desktops and see if the problem goes away.
Please observe that there is a serious backside on using this workaround, anyone that have console access to vCenter/ESX can see the display of the desktop and that might be a problem with some users.
// Linjo
